The present invention relates generally to a variable speed drive for a chiller system. More specifically, the present invention relates to a variable speed drive with boost capability for a variable speed chiller system with a switched reluctance motor.
In the past, the induction motors for driving compressors in chiller systems were designed to operate from standard line (main) voltages and frequencies that were available from the power distribution system of the facility where the motor was being operated. The use of line voltages and frequencies typically required the compressors to use some inefficient mechanical means (such as inlet guide vanes for centrifugal compressors and a slide valve for screw compressors) for modulating capacity as a result of the motor being limited to one operating speed that was based on the input frequency to the motor. In addition, if the operating speed of the motor was not equal to the desired operating speed of the compressor, a “step up,” or “step down,” gearbox was inserted between the motor and the compressor to obtain the desired operating speed of the compressor. Furthermore, motors that required their own controller or electronic drive, e.g., switched reluctance motors, could not be used for these chiller systems, as such motors could not operate directly from standard (main) voltages and frequencies.
Next, variable speed drives (VSDs) were developed that could vary the frequency and/or voltage that was provided to the induction motors of a chiller system. This capability to vary the input frequency and voltage to a motor resulted in an induction motor that was capable of providing a variable output speed and power to the corresponding compressor of the chiller system. The variable speed operation of the motors (and compressors) enabled the chiller system to take advantage of efficiencies that occur during partial loading of the compressors, when operation at a speed lower than full load design speed is desirable. The use of the variable speed drive also permitted the use of other types of motors that required their own electronic drive, e.g., switched reluctance motors, in chiller systems in addition to the previous motors that were capable of operating directly from a three-phase power line, e.g., induction motors or synchronous motors.
One limitation of prior induction motor style VSDs is that the magnitude of the output voltage from the VSD can be no larger than the magnitude of the input, or utility, line voltage to the VSD. This limit on the output voltage occurs because the rectifier of the VSD only provides a DC voltage that is at a magnitude equal to approximately 1.3 times the root mean square (rms) value of the line-to-line AC voltage supplied to the VSD. This limitation on the output voltage of the variable speed drive limits the maximum speed of the conventional induction motor to a speed that corresponds to the speed of the motor operated at line voltage (because of the constant volts/hertz ratio required by a conventional induction motor). To obtain greater compressor speeds, a “step up” gearing arrangement has to be incorporated between the motor and the compressor to increase the output rotational speed of the motor driving the compressor. Alternately, one could use a lower rated voltage motor and operate the motor at higher than its rated voltage and frequency to obtain higher maximum rotational speed, provided the motor was physically capable of such high-speed operation. In this regard, the switched reluctance motor has a distinct advantage over the induction motor because the switched reluctance motor is able to operate at higher rotational speeds due to the physical simplicity of the motor rotor construction.
In addition, this limitation on the output voltage from the VSD limits the operating speed range of high speed motors, including high speed switched reluctance motors, in the chiller system. The high speed motors, used to obtain faster compressor speeds without the “step up” gearing arrangement, are limited because it is more difficult to design an efficient and cost-effective motor when only a limited voltage range is available. High speed switched reluctance motors are desirable in a chiller system because they are capable of higher efficiency, improved reliability, and lower cost, than other types of motors. In addition, the physical simplicity of the rotor construction of the switch reluctance motor lends itself to a higher degree of mechanical robustness, providing for ease of use in high speed applications.
Therefore, what is needed is a variable speed drive for a high speed switched reluctance motor that can provide a cost-effective, efficient and easily implemented operation of the high speed switched reluctance motor in a chiller system.